FIELD OF THE INVENTION
The invention relates to a once-through steam generator with a combustion chamber surrounded by a containment wall composed of tubes connected to one another in a gastight manner, a flow medium being capable of flowing from the bottom upward through the vertically extending tubes which have a surface structure on their inside. It relates, further, to a method for configuring a once-through steam generator of this type.
A steam generator of this type is known from the paper "Verdampferkonzepte fur Benson-Dampferzeuger" [Evaporator Concepts for Benson Steam Generators] by J. Franke, W. Kohler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, pages 352-360. In a once-through steam generator of this type, in contrast to a natural circulation or forced circulation steam generator with only partial evaporation of the water/steam mixture, the heating of evaporator tubes forming the combustion chamber leads to the complete evaporation of the flow medium in the evaporator tubes in a single pass. Whereas, in the natural circulation steam generator, the evaporator tubes extend substantially, the evaporator tubes of the once-through steam generator may be both vertically and spirally, hence at an incline.
A once-through steam generator, the combustion chamber walls of which are composed of vertically arranged evaporator tubes, can be produced more cost-effectively than a once-through steam generator having spiral tubing. Furthermore, once-through steam generators with vertical tubing have lower water-side/steam-side pressure losses than those with evaporator tubes which are inclined or that ascend spirally. Furthermore, in contrast to a natural circulation steam generator, a once-through steam generator is not subject to any pressure limitation, so that fresh steam pressures well above the critical pressure of water (p.sub.crit =221 bar), where there is only a slight density difference between the liquid-like and steam-like medium. High fresh-steam pressures are necessary in order to achieve high thermal efficiencies and consequently low CO.sub.2 emissions.
A particular problem, in this case, is the structural design of the combustion-chamber or containing wall of the once-through steam generator with regard to the operating temperatures of the tube-wall or the materials. In the subcritical pressure range up to about 200 bar, the temperature of the combustion-chamber wall is determined essentially by the value of the water saturation temperature, when wetting of the heating surface in the evaporation zone can be ensured. This is achieved, for example, by the use of internally ribbed tubes. Tubes of this type and their use in steam generators are known, for example, from European patent application 0 503 116. These so-called ribbed tubes, i.e., tubes with a ribbed inner wall surface, have particularly good heat transfer from the inner wall to the flow medium.
In the pressure range of about 200 to 221 bar, the heat transfer from the inner wall of the tube to the flow medium decreases sharply, so that the flow velocity--the mass flow density is usually used as a measure of this--has to be increased correspondingly, in order to ensure that the tubes are cooled sufficiently. Consequently, in the evaporator tubes of once-through steam generators operated at pressures of approximately 200 bar and above, the mass flow density and therefore the pressure loss due to friction must be higher than in once-through steam generators operated at pressures of below 200 bar. Particularly in the case of small inner diameters of the tubes, the higher pressure loss due to friction cancels out the advantageous property of vertical tubing but, when there is multiple heating of individual tubes, their through put also rises. However, since high steam pressures of more than 200 bar are required in order to achieve high thermal efficiencies and therefore low CO.sub.2 emissions, it is necessary, in this pressure range too, to ensure good heat transfer. Consequently, once-through steam generators with a vertically tubed combustion-chamber wall are conventionally operated with relatively high mass flow densities in the tubes, so as to ensure, in the unfavorable pressure range of about 200 to 221 bar, that there is always a sufficiently degree of heat transfer from the tube wall to the flow medium, that is to say to the water/steam mixture. In this context, the publication "Thermal Engineering" I. E. Semenovker, Vol. 41, No. 8, 1994, pp. 655-661, specifies a mass flow density at 100% load of about 2000 kg/m.sup.2 s consistently both for gas-fired and for coal-fired steam generators.